1. Field of the Invention
The present invention relates to systems and methods of mounting in-plane sensors, and more particularly to systems and methods of mounting in-plane sensors on two orthogonal surfaces to form a three-axis inertial measurement unit having tetrahedral angle redundancy.
2. Description of Related Art
Inertial measurement units (IMU) are known in the art and have been used in a wide variety of applications. For example, IMUs are commonly used in inertial guidance and navigation systems for all types of vehicles, in particular aircraft and spacecraft. Inertial navigation has the advantage of not being dependent on an external point of reference. Navigation is accomplished by sensing the motion of the vehicle and calculating the change in position with respect to an initial position. The IMU is able to determine the three-dimensional orientation of a body relative to a reference direction absolutely within an inertial system.
A typical IMU may consist of three equal modules, each including a gyroscopic rotational rate sensor, a linear accelerometer, and associated electronics. Each module is typically oriented on a cube or a similar structure to provide inertial measurements along one of three orthogonal axes, with the gyroscopic rotational rate sensors providing information regarding rotation of the unit and the accelerometers providing information concerning linear movement of the unit. In this way, the IMU is able to determine the position of the vehicle with respect to the vehicle's initial position to aid in guidance, navigation, and control of the vehicle.
Three-axis inertial measurement units as described above have been used extensively in aerospace applications. Traditionally, such IMUs included mechanical sensors such as conventional spinning mass gyroscopes and large mechanical accelerometers. However, most current IMUs utilize microelectromechanical systems (MEMS) devices. Many MEMS sensors are mounted on a support substrate made of silicon or a similar material and can detect acceleration by measuring a change in capacitance. Current technologies using MEMS devices encapsulate the accelerometer, gyroscope, and associated electronics into individual packages. These packages are typically soldered to a circuit board, which is then mounted on one plane of an orthogonal assembly, such as a face of a cube.
Most inertial sensors, including MEMS sensors, are perpendicular sensors or out of plane devices, meaning that the sense axis of the device is oriented at a 90 degree angle with respect to the mounting plane. Some MEMS devices, including accelerometers and gyroscopes, are in-plane sensors. In-plane sensors are inertial sensors having a sense axis that is parallel to the mounting plane. In-plane sensors detect an acceleration or rotation along an axis parallel to the surface of the support substrate.
Redundant systems of out of plane sensors arranged along non-orthogonal axes are well known in the art. Redundant inertial measurement systems allow for the failure of one or more sensors while still maintaining the ability to determine the essential location of a vehicle in inertial space. Prior art systems have utilized a variety of structures for mounting out of plane sensors to produce redundant systems. For example, out of plane sensors have been placed on the orthogonal faces of a cube, on the faces of a pyramid or wedge, on the faces of a dodecahedron, and on a tetrahedral structure.
A tetrahedral structure is particularly advantageous for redundant systems. In this configuration, four out of plane sensors are mounted to the four faces of an equilateral tetrahedron, with each of the sensors located symmetrically with respect to the others at an angle of approximately 109.4 degrees between the sense axes of the sensors. This configuration simplifies the equations and resultant equipment necessary to isolate failed sensors and convert output signals from the sensors to a coordinate system fixed in inertial space.
Although a tetrahedral structure is advantageous for redundant inertial measurement units, such a shape adds complexity to the inertial measurement unit and thus increases costs of manufacturing the unit. Accordingly, there is a need in the art for an inertial measurement unit that can provide the advantages of a tetrahedral configuration without the added complexity required by such a configuration.